Guided elastic waves have a great potential in pipe inspection as an efficient and low-cost nondestructive evaluation （NDE） technique, among which the wave of mode L（0, 2） receives a lot of attention because this ...Guided elastic waves have a great potential in pipe inspection as an efficient and low-cost nondestructive evaluation （NDE） technique, among which the wave of mode L（0, 2） receives a lot of attention because this mode is the fastest mode in a weakly dispersive region of frequency to minimize dispersion effects over a long distance and sensitive to the defects distributed circumferentially. Though many experimental and numerical researches have already been carried out about the excitation of L（0, 2） and its interaction with the defect in a hollow cylinder, its excitation mechanism has not been clarified yet. In this paper based on the transient response solution of the hollow cylinder, derived by the method of eigenfunction expansion, the theory about the exciting mechanism of mode L（0, 2） is advanced and the effects of the spatial distribution, vibration frequency and direction of the external force on the excitation are discussed. And the pure mode L（0, 2） is excited successfully under the parameters obtained through theoretical analysis. Furthermore, its interactions with some kinds of defects in hollow cylinders are simulated with the method of finite element analysis （FEA） and the results agree well with those obtained by other researchers.展开更多
The [001]c-polarized(1-x)Pb(Mg1/3Nb2/3)O3–xPbTiO3(PMN-PT) single crystals are widely used in ultrasonic detection transducers and underwater acoustic sensors. However, the relatively small coercive field( 2 kV/cm) of...The [001]c-polarized(1-x)Pb(Mg1/3Nb2/3)O3–xPbTiO3(PMN-PT) single crystals are widely used in ultrasonic detection transducers and underwater acoustic sensors. However, the relatively small coercive field( 2 kV/cm) of such crystals restricts their application at high frequencies because the driving field will exceed the coercive field. The depolarization field can be considerably larger in an antiparallel direction than in a parallel direction with respect to polarization when the bipolar driving cycle starts. Thus, if the direction of the sine wave signal in the first half cycle is opposite to the polarization direction, then the depolarized domains can be repolarized in the second half of the sine cycle. However, if the direction of the sine wave signal in the first half of the cycle is along the polarization direction, then the change is negligible,and the domains switched in the second half of the sine cycle cannot be recovered. The design of electric driving method needs to allow the use of a large applied field to emit strong enough signals and produce good images. This phenomenon combined with the coercive field increases with the driving frequency, thereby making the PMN-PT single crystals usable for high-frequency applications. As such, the applied field can be considerably larger than the conventionally defined coercive field.展开更多
A spherical cap radiator is one of the important parts of an underwater wide-beam imaging system. The back radiation of a traditional spherical cap radiator, which is composed of a vibrating cap and a rigid baffle, is...A spherical cap radiator is one of the important parts of an underwater wide-beam imaging system. The back radiation of a traditional spherical cap radiator, which is composed of a vibrating cap and a rigid baffle, is strong and its far-field directivity function may fluctuate in big amplitude in the vicinity of the polar axis. These shortcomings complicate the processing of the reflective waves received for imaging the targets. In this study, the back radiation is weakened by adding an acoustic soft material belt between the vibrating cap and the rigid baffle. And the fluctuation mentioned above is lowered remarkably by dividing the spherical cap radiator into many annuluses and a relatively smaller spherical cap, and by controlling the phase retardations of all elements appropriately. Furthermore, the numerical experiments are carried out by the finite element method （FEM） to prove the validity of the above methods.展开更多
The microstructure effect of ultrasonic waves in a unidirectional titanium graphite composite is analyzed by the mode energy conversions of the laser-generated ultrasonic Lamb wvaves.The carrying energy of each mode i...The microstructure effect of ultrasonic waves in a unidirectional titanium graphite composite is analyzed by the mode energy conversions of the laser-generated ultrasonic Lamb wvaves.The carrying energy of each mode in the Lamb wvaves is calculated quantitatively by the time-frequency filtering technique of the Wigner distribution.We found that the energy conversions among modes have happened in the process of propagation of ultrasonic Lamb wave.These energy conversions are attributed to microstructure scattering of ultrasonic wave by the fibers in a fiber reinforced composite material.This work will provide a quantitative method of ultrasonic characterizationof microstructure feature of the composite materials by the laser-generated Lamb wave technique.展开更多
文摘Guided elastic waves have a great potential in pipe inspection as an efficient and low-cost nondestructive evaluation （NDE） technique, among which the wave of mode L（0, 2） receives a lot of attention because this mode is the fastest mode in a weakly dispersive region of frequency to minimize dispersion effects over a long distance and sensitive to the defects distributed circumferentially. Though many experimental and numerical researches have already been carried out about the excitation of L（0, 2） and its interaction with the defect in a hollow cylinder, its excitation mechanism has not been clarified yet. In this paper based on the transient response solution of the hollow cylinder, derived by the method of eigenfunction expansion, the theory about the exciting mechanism of mode L（0, 2） is advanced and the effects of the spatial distribution, vibration frequency and direction of the external force on the excitation are discussed. And the pure mode L（0, 2） is excited successfully under the parameters obtained through theoretical analysis. Furthermore, its interactions with some kinds of defects in hollow cylinders are simulated with the method of finite element analysis （FEA） and the results agree well with those obtained by other researchers.
基金Project supported by the National Natural Science Foundation of China(Grant No.11674270)the Fundamental Research Funds for Xiamen University,China(Grant No.20720180113)+2 种基金the Education and Scientific Research Project for Young and Middle-aged Teachers of Fujian Province,China(Grant No.JAT170036)the Opening Fund of Acoustics Science and Technology Laboratory,China(Grant No.SSKF2018006)sponsored by the Education Department of Fujian Province,China for his study at the Pennsylvania State University(Grant No.2016071145)
文摘The [001]c-polarized(1-x)Pb(Mg1/3Nb2/3)O3–xPbTiO3(PMN-PT) single crystals are widely used in ultrasonic detection transducers and underwater acoustic sensors. However, the relatively small coercive field( 2 kV/cm) of such crystals restricts their application at high frequencies because the driving field will exceed the coercive field. The depolarization field can be considerably larger in an antiparallel direction than in a parallel direction with respect to polarization when the bipolar driving cycle starts. Thus, if the direction of the sine wave signal in the first half cycle is opposite to the polarization direction, then the depolarized domains can be repolarized in the second half of the sine cycle. However, if the direction of the sine wave signal in the first half of the cycle is along the polarization direction, then the change is negligible,and the domains switched in the second half of the sine cycle cannot be recovered. The design of electric driving method needs to allow the use of a large applied field to emit strong enough signals and produce good images. This phenomenon combined with the coercive field increases with the driving frequency, thereby making the PMN-PT single crystals usable for high-frequency applications. As such, the applied field can be considerably larger than the conventionally defined coercive field.
基金Project supported by the National High Technology Research and Development Program of China (Grant No. 2006AA09Z109)the Natural Science Foundation of Fujian Province,China (Grant No. T0750014)
文摘A spherical cap radiator is one of the important parts of an underwater wide-beam imaging system. The back radiation of a traditional spherical cap radiator, which is composed of a vibrating cap and a rigid baffle, is strong and its far-field directivity function may fluctuate in big amplitude in the vicinity of the polar axis. These shortcomings complicate the processing of the reflective waves received for imaging the targets. In this study, the back radiation is weakened by adding an acoustic soft material belt between the vibrating cap and the rigid baffle. And the fluctuation mentioned above is lowered remarkably by dividing the spherical cap radiator into many annuluses and a relatively smaller spherical cap, and by controlling the phase retardations of all elements appropriately. Furthermore, the numerical experiments are carried out by the finite element method （FEM） to prove the validity of the above methods.
基金Supported by the National Natural Science Foundation of China under Grant No.19974017and the Doctoral Foundation of the Education Ministry of China under Grant No.1999028432.
文摘The microstructure effect of ultrasonic waves in a unidirectional titanium graphite composite is analyzed by the mode energy conversions of the laser-generated ultrasonic Lamb wvaves.The carrying energy of each mode in the Lamb wvaves is calculated quantitatively by the time-frequency filtering technique of the Wigner distribution.We found that the energy conversions among modes have happened in the process of propagation of ultrasonic Lamb wave.These energy conversions are attributed to microstructure scattering of ultrasonic wave by the fibers in a fiber reinforced composite material.This work will provide a quantitative method of ultrasonic characterizationof microstructure feature of the composite materials by the laser-generated Lamb wave technique.
